Carbon nanotubes (CNTs) are self-assembling nanostructures comprised of graphite sheets rolled up into cylinders. Such nanostructures are termed single-walled carbon nanotubes (SWNTs) if they are comprised of a single cylindrical tube. CNTs comprising two or more concentric tubes are termed double-walled carbon nanotubes (DWNTs) and multi-wall carbon nanotubes (MWNTs), respectively. The diameters of SWNTs typically range from 0.4 nm to ca. 3 nm, and the lengths from ca. 10 nm to a few centimeters.
Whether made by laser ablation, chemical vapor deposition or other techniques, the carbon nanotubes often contain catalyst residues from their synthesis. In many applications, those catalyst residues are detrimental to the final end-use application.
A variety of methods for de-metallization of carbon nanotubes have been developed, but they typically rely upon the use of oxidizing conditions in strong acids. Such conditions will remove much of the catalyst residue, and will even remove carbon species such as amorphous carbon. However, these aggressive processes can also result in severe damage to, and loss of, CNTs.
Chinese patent application CN1485271A discloses a method to remove Co, Ni and Fe in CNTs by heating them under hydrogen to 650° C. and then CO gas at 150-200° C. before removing the metals under vacuum.
A non-destructive mild oxidation method of removing some impurities from as-grown carbon nanotubes, including single-wall carbon nanotubes and multi-wall carbon nanotubes, by H2O2 oxidation and HCl treatment has been investigated (Feng et al., Chem. Phys. Lett. 2003, 375, 645-648).
An efficient, industrial scale purification process to remove these impurities is desired, as many of the applications of CNTs require highly-purified CNTs with low levels of damage to the CNT structure. A high-yield method that removes the catalyst residues from the carbon nanotubes, that does not require strong acid oxidation, and that leaves the carbon nanotubes in an unoxidized form, would be of considerable value.